Urea evaporation initial startup

Two phases of initial operation for evaporation: First, remember one principle – when starting up, raise the temperature first and then create a vacuum; when shutting down, release the vacuum first and then lower the temperature. Prevent crystalline blockage. Everyone understands it in theory, but it’s still chaotic the first time you do it; P2: when the temperature drops, you must first start the circulation cycle before breaking the vacuum. Otherwise, the system will be in complete disarray after diarrhea. 3 The evaporation system also requires a positive pressure hold test to check for leaks. 4 Before driving the machine, use deionized water to flush the system and conduct a vacuum pumping test. 5 If possible, physical granulation experiments should also be conducted to test the evaporation system and the granulation conveying system. 6 Several level gauges need to be calibrated, which has a significant impact on the initial startup process. 7 Even when several pumps are evaporating, the pump should still function properly when tested with water. These are some of our initial experiences with starting evaporation; we hope they can be of some reference to you. As for the many problems that have arisen, they need to be analyzed on a case-by-case basis; we can discuss them further later. ;P Last edited by lxq700918 on 2009-2-11 23:11 ]

A lot has already been said on the 2nd floor, and I would like to add the following here: 1. All the material pipelines in the evaporation system must be at a certain temperature before materials can be fed into them; this is essential. In the past we encountered a situation where, although the pipelines were preheated using condensate, no sufficient amount of water was added after the melting pump was started, which led to blockages in the main granulation line, forcing us to shut down the system and build a new one； 2. It is essential to ensure that the liquid drainage from all surface coolers is unobstructed. We use a production system for large-grain urea that relies on CO2 stripping combined with fluidized-bed granulation; in this system, blockages have occurred in the liquid drainage pipes of the flash condenser, which ultimately resulted in the evaporation vacuum level not being sufficient for production after the system was put into operation ； 3. There is also another requirement: when the system is initially started up, the insulation steam for all evaporation pipelines must be turned on. For example, the insulation steam for the pipelines used for liquid discharge must be enabled; otherwise, crystallization in those liquid discharge pipelines can occur. Well, that’s all for the additions! In general, a lot of experience is accumulated through actual production operations. The original poster is very lucky; I’m not sure if what we’ve said will be useful to you. Hehe, it’s still necessary to keep summarizing things in actual production! ! ! !

Points to note when initially starting up the evaporation unit: 1. Fill the liquid level tanks before starting up; this is done to ensure proper liquid sealing in each tank. In addition, water is filled into the downflow pipe from the flash tank to the vaporizer U-tube, as well as the U-tube between stage 1 and stage 2. This prevents vacuum from connecting the flash tank to stage 1, and stage 1 to stage 2, thereby facilitating the creation of a vacuum difference that allows the material to enter the system smoothly. At the same time, the steam condensate in the U-tube also preheats the pipes, diluting the urine that has just entered the system and preventing crystallization blockages before the temperature reaches normal levels. Before the output from the urea synthesis tower, water should also be injected into the heat utilization section at the bottom of the first-stage evaporator heater (usually, water is already supplied to this area during system filling operations), in order to prevent ammonium methyl sulfate crystals from forming as a result of the condensation of the pre-distillation gas. 2. Check the primary and secondary surface coolers, as well as the condition of the circulating cooling water supplied to the intercooler. 3. Understand the pressure in the steam main, steam saturation, and the amount of steam that can be supplied. Check whether the drainage systems of the two evaporation heaters are unobstructed. Before starting the operation, it is necessary to properly open the bypass valve of the steam trap in order to discharge the excess steam condensate during startup, thereby facilitating heating in the first and second stages. 4. Feed steam at 0.28 MPa into the insulation steam jacket or accompanying pipe to check the preheating of the tubes, and fill the urine tank with water to a 20% liquid level. 5. Determine the discharge time from the synthesis tower; after discharge, increase the vacuum level slightly and maintain the vacuum degree in the flash tank at 0.02 MPa. The freshly discharged dilute urine is stored in the urine tank, and water is sent to the first-stage evaporation unit to help utilize the heat generated by ammonium formation in that stage. 6. One hour before the discharge from the synthesis tower, at this point after the second-stage decomposition, the urine assumes normal composition (that is, the concentration of the happy liquid increases, and the levels of free ammonia and other components are normal). (1) Water is fed into the first stage of the evaporation heater, which slightly raises the temperature and vacuum level in the first and second stages. The water from the second stage exits at the inlet of the melting pump and is discharged into the drain channel (with the liquid level maintained as indicated by the gauge in the pipeline). This process serves the purpose of evaporation; the evaporation heater is used to preheat the system equipment and pipelines, as well as to check for any issues within the system. (2) If the melt pump needs maintenance or its condition is unknown, the pump can be tested by feeding water into the main line; the water should flow out from the nozzle. The testing time should not be too long, and once it is confirmed that the auxiliary line for testing is unobstructed, the pump should be stopped, with the water continuing to flow out through the pump’s inlet discharge valve. (3) Blow steam into the main granulation pipe to start the nozzles in operation. (4) One hour after the discharge from the synthesis tower, it was confirmed that the temperature for the second-stage decomposition was normal; the vacuum-fed liquid from the flash tank was then fed into the first-stage evaporation heater, the water flow was stopped, and the valve leading to the urine collection tank was closed (with low-pressure steam being used to purge this pipeline). (5) Adjust the temperatures and vacuum levels of stage 1 and stage 2, start the melting pump, and the urine enters the urine tank through the circulation pipeline. (6) When the processes in stages one and two are approaching normal operation, notify the packaging team to start the belt conveyor and prepare for packaging. (7) For the safety of the granulation process, when the vacuum level in the second stage reaches 0.08 MPa, granulation can begin at a temperature of 140°C. In this case, it is possible to maintain a slightly higher liquid level and open the feed valve to a greater degree in order to increase the speed of the material through the pipes. Once particles are formed, the vacuum level should be increased to above 0.09 MPa, and the main valve should be adjusted to its normal opening degree. 7. Once the evaporation process conditions and granulation parameters are normal, start the urine pump and add a small amount of urine from the urine tank that was unsuitable for use during startup into the evaporation system (without affecting the evaporation process conditions), until the required amount is reached (up to a liquid level of 20%). Note: During the initial evaporation process, it is not possible to use urine that does not meet the standards; this is because both the area of the heater and the amount of steam supplied are calculated based on urine of standard quality. When urine that does not meet the standards is used, neither the temperature nor the vacuum level can be increased. If circulation continues for a long time, the level of biuret in the urine rises significantly, resulting in a large number of defective products. Moreover, it is prone to accidents such as crystalline blockages during operation.

“After enabling the melt pump, an appropriate amount of water was not added, which caused blockage in the main granulation line; as a result, the machine had to be stopped to prepare a new main granulation line”； Why is it necessary to add an appropriate amount of water? Where should water be added? Should the nozzles be preheated with steam before granulation? ? Could you please explain it to me?

““Furthermore, water is filled in the downflow tube from the flash tank to the steam heater’s U-tube, as well as in the U-tube between stage 1 and stage 2.” Hello, I’m a beginner; what is the purpose of these two U-tubes? Are there any requirements for pipeline layout? How is the vacuum level between the different stages achieved? A vacuum pump creates negative pressure; aren’t the vacuum levels in different stages different? {:1_90:}

The function of the U-tube is to maintain the pressure difference between the two sections. If fluid moves directly from stage one to stage two, the low pressure in stage two prevents the liquid level from being maintained, allowing gas to escape into stage two. Production cannot be maintained either. To allow liquid to enter the second stage while preventing gas from doing so, the length of the pipeline must be increased. That’s why a U-tube was installed. Its height is calculated to ensure a maximum pressure difference between section 1 and section 2; it’s not something that can be set arbitrarily.